The present invention relates to transparent glass ceramics, in particular, to substantially transparent glass-ceramics based on a predominant crystal phase of forsterite.
Glass-ceramics are polycrystalline materials formed by a controlled crystallization of a precursor glass. The method for producing such glass-ceramics customarily involves three fundamental steps: first, a glass-forming batch is melted; second, the melt is simultaneously cooled to a temperature at least below the transformation range thereof and a glass body of a desired geometry shaped therefrom; and third, the glass body is heated to a temperature above the transformation range of the glass in a controlled manner to generate crystals in situ.
Frequently, the glass body is exposed to a two-stage treatment. Hence, the glass will be heated initially to a temperature within, or somewhat above, the transformation range for a period of time sufficient to cause the development of nuclei in the glass. Thereafter, the temperature will be raised to levels approaching, or even exceeding, the softening point of the glass to cause the growth of crystals on the previously-formed nuclei. The resultant crystals are commonly more uniformly fine-grained and the articles are typically more highly crystalline. Internal nucleation allows glass-ceramics to possess such favorable qualities as a very narrow, particle size distribution and highly uniform dispersion throughout the glass host.
Transparent glass-ceramics are well known to the art; the classic study thereof being authored by G. H. Beall and D. A. Duke in xe2x80x9cTransparent Glass-Ceramicsxe2x80x9d, Journal of Materials Science, 4, pp. 340-352 (1969). Glass-ceramic bodies will display transparency to the human eye when the crystals present therein are considerably smaller than the wavelength of visible light. More specifically, transparency generally results from crystals less than 50 nm, and preferably as low as 10 nm, in size. Transparency in glass-ceramics can also be produced with crystals larger than 50 nm if the crystal birefringence and the index of refraction mismatch between the crystal phase and the glassy phase are both low.
Recently, much effort has been concentrated in the area of using transparent glass-ceramics as hosts for transition metals which act as optically active dopants. Suitable glass-ceramic hosts must be tailored such that transition elements will preferentially partition into the crystals. Co-pending application Ser. No. 60/160,053, entitled xe2x80x9cTransition Metal Glass-Ceramics,xe2x80x9d by Beall et al., is co-assigned to the present assignee, and is herein incorporated by reference in its entirety. It is directed at transition-metal doped glass-ceramics suitable for formation of a telecommunications gain or pump laser fiber.
Transparent glass-ceramics, which contain a relatively small volume percentage of crystals, can be of great use in cases where the parent glass provides an easy-to-melt or an-easy-to-form vehicle for a crystal. The crystal, in itself, may be difficult or expensive to synthesize, but may provide highly desirable features, such as optical activity. The crystals in the glass-ceramic are generally oriented randomly throughout the bulk of the glass, unlike a single crystal which has a specific orientation. Random orientation, and consequent anisotropy, are advantageous for many applications, one example being that of optical amplifiers, where polarization-independent gain is imperative.
Transparent glass-ceramics, doped with transition elements, can combine the optical efficiency of crystals with the forming flexibility of glass. For example, both bulk (planar) and fiber forms can be fabricated from these glass-ceramics.
Therefore, there exists a need for transparent, glass-ceramic materials which contain small tetrahedral and interstitial sites, and hence are suitable as potentially valuable hosts for small, optically active, transition elements. Such elements include, but are not limited to, Cr4+, Cr3+, Co2+, Cu2+, Mn2+, Ni2+, Fe3+, Fe2+, and Cu+, which impart luminescence and fluorescence thereto. The doped glass-ceramic materials are suitable for application in the optical field industry.
In the late 1980s, it was discovered that chromium-doped, forsterite, single crystals could be used as a laser material in the 1210 nm to 1260 nm range. Further work determined that the active ion was Cr4+, a rare valence state of chromium, and that strong luminescence and tunable laser action could be expected in the broad spatial region from 1.1xcexc to 1.4xcexc, and perhaps deeper into the infrared.
U.S. Pat. No. 5,717,517 is directed at a method for amplifying a signal pulse of a laser light by providing an amplifying medium which contains an elongated core having crystalline particles dispersed within a non-gaseous medium. Cr4+ doped forsterite single crystals are provided as an example of a suitable crystalline particle.
However, what the prior art has failed to disclose, and what this invention teaches, is a forsterite glass-ceramic that is substantially transparent and suitable for employment in the fiber optics industry.
Accordingly, the primary object of the present invention is to provide glass-ceramic materials which are substantially, and desirably totally, transparent, and which contain a predominant crystal phase of forsterite.
Another object of the present invention is to provide such forsterite glass-ceramics which are capable of being doped with ingredients which confer luminescence and/or fluorescence thereto.
An important advantage of the present glass-ceramic family is that it provides a material containing forsterite crystals which selectively incorporate transition metal ions including, but not limited to Cr4+, Cr3+, Co2+, Cu2+, Mn2+, Ni2+, Fe3+, Fe2+, and Cu+. The material is glass-based, thus providing the important flexibility of allowing for fabrication of both bulk (such as planar substrates) and fiber (such as optical fiber) forms.
Other objects and advantages of the present invention will be apparent from the following description.
In accordance with the present invention, there is provided a transparent glass-ceramic containing a predominant crystal phase of forsterite and having a composition consisting essentially of, in weight percent on an oxide basis, SiO230-60; Al2O3 10-25; MgO 13-30; K2O 8-25; TiO2 0-10; and GeO2 0-25.
The most preferred composition range, for best transparency, is based on a composition consisting essentially, in weight percent on an oxide basis, of SiO2 43-55; Al2O3 12-20; MgO 15-25; K2O 10-17; and TiO2 3-8. To obtain optical activity, i.e., fluorescence, over the communications transmission wavelength range of 1100 to 1700 nm, the present inventive, forsterite glass-ceramics are doped with up to 1 wt. % chromium oxide, and preferably with 0.003 to 0.3 wt. % chromium oxide.
A method of making the inventive glass-ceramic materials is also provided comprising the steps of:
a.) melting a batch for a glass having a composition consisting essentially, in weight percent on an oxide basis, of SiO2 30-60; Al2O3 10-25; MgO 13-30; K2O 8-25; TiO2 0-10; and GeO2 0-25.
b.) cooling the glass to a temperature at least below the transformation range of the glass;
c.) exposing the glass to a temperature between about 600-1000xc2x0 C. for a period of time sufficient to cause the generation of a glass-ceramic which is substantially transparent and which contains a predominant crystal phase of forsterite; and,
d.) cooling the glass-ceramic to room temperature.